It is well established that the benefits of various turbine engines vary with the speed range in which they operate. While the turbojet engine is generally more satisfactory for transonic and supersonic speeds, the high-bypass-ratio turbofan engine is more desirable for subsonic flight speeds because of its higher efficiency at such speeds and its lower noise level in the vicinity of the airport.
The wide interest in a commercially operable supersonic transport aircraft (SST) has pointed out the need for meeting the combined criteria of low airport noise level and high operating efficiency in both subsonic and supersonic flight. Various proposals have been made for dual-cycle propulsion systems which combine the features of the turbojet and turbofan engine concepts. One possible approach is the use of two virtually independent sets of engines with each set operating primarily only during the portion of the flight when its characteristics are most desirable. In this method one set of engines is substantially unused at all times and as a result the total propulsion installation suffers a severe penalty in weight and bulk. A second approach is the use of a moderate to high bypass ratio turbofan with internal variable geometry features that permit a substantial reduction in bypass ratio for high-speed operation. The disadvantages of this second approach are that the engine cycle cannot provide supersonic cruise efficiency as high as that for a pure turbojet and, at the same time, the supersonic drag of the installation is increased because the air inlet must be sized for low speed operation. The present invention solves the weight, bulk, and performance problems of the two approaches just discussed through an air interchange concept which accomplishes a complete conversion from a turbojet to a turbofan cycle with all components of the engine utilized at all times, and permits the supersonic air inlet to be sized for the supersonic case only.